Why is Counter-Strike 2 using the wrong microphone on PC?

Voice chat in Counter-Strike 2 is acting dead even though the headset works everywhere else. There is no clear error, and switching push-to-talk on and off does not change much.

What is the fastest way to separate a game setting issue from a Windows input-device issue? The game-specific side includes Reflex, fullscreen mode, shader cache, and frame-time stability.